Practical methods for the morphological recognition and definitionof genera, with a comment on polychaetes (Annelida)

Abstract

There are very few publications dealing with methods for the morphological recognition of genera, and how they can be defined, in comparison to those available dealing with species issues. My objective is to provide a historical review, synthesize and discuss some ideas or practical procedures about this problem. Genera are recognized because member species depict a general morphological pattern, and usually one or a few diagnostic characters separate each genus from other similar genera. Human mind detects patterns by comparative morphology and this explains why experience is extremely important in taxonomy. Analogy is also involved, because by understanding how character patterns help recognizing taxonomic groups, these patterns can be extrapolated in less well-known groups. From an historical perspective, botanists and zoologists perceived or defined genera differently with some common considerations and procedures. Genera are natural groups, size-variable and shape-conservative, that are recognized by different cultures. As explanatory hypothesis, genera are unstable and difficult to define because their contents are modified after the study of species from different localities; once planetary revisions are made, the resulting delineation is improved because variations are better understood or assimilated into current definitions. A necessary step for this improvement is the standardization of the terminology for morphologic features, but planetary revisions are the only means to reach this goal. As in other fields in systematic zoology, the recognition of genera among marine annelids (polychaetes) relied in a comparative approach, after the standardization of the terminology for body appendages. The study of larger collections with specimens from distant localities helped to clarify the morphological patterns, but their evaluation sometimes drove to contradictory conclusions, such as a widespread acceptance of cosmopolitan species. Although there are several pending issues, there has been a progressive improvement, especially after the inclusion of additional methods, but more efforts are needed for taxonomic training, and for improving the job market.